1. Field of the Invention:
This invention relates to an apparatus for sensing vehicular mechanical motion, which finds particular, but not exclusive, utility in the environment of rail mass transit systems.
2. Description of the Prior Art:
An unauthorized or unexpected motion in modern rail transit systems may cause an unsafe condition in which the physical well being of passengers using the rail system is placed in peril. For example, a potentially fatal unsafe condition occurs upon the movement of a rail vehicle, due to the accidental release or failure of the vehicle brakes, while passenger doors of the rail vehicle are for some reason open. In that event, it is imperative to quickly detect the existence of such an unsafe condition, and to take immediate effective action to rectify the situation. Thus, most modern rail-transit systems incorporate tachometric sensors to detect vehicle motion and to produce a signal indicative thereof, by which predetermined measures can be taken to protect against an unsafe condition. However, tachometric sensors tend to be relatively expensive, and their use can therefore be financially prohibitive, especially where large numbers of sensors are required.
Most modern rail transit systems employ cycle checking safety design techniques to protect against unsafe conditions. Cycle checking involves the continuous testing of a device, circuit, or computer instruction to insure that it is completely functional. Furthermore, safety design techniques are generally directed to the promotion of fail-safe, or "vital", operation, in which any failures which occur tend to result in a condition which is no more dangerous (or conversely at least as safe) as if an equipment failure had not occured.